FOUS11 KWBC 022022
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
322 PM EST Thu Jan 2 2025

Valid 00Z Fri Jan 03 2025 - 00Z Mon Jan 06 2025

...The West...=20
Days 1-3...

The primary weather maker in the short term is a potent 500mb low
approaching from the west that will promote falling heights,
lowering snow levels, and tap into a healthy plume of Pacific
moisture over the northwestern U.S. starting this evening. NAEFS
shows by 12Z Friday that 500mb heights are dropping below the 10th
climatological percentile off the Pacific Northwest coast, while
>500 kg/m/s IVT values (which top the 90th climatological
percentile) are direct over northern California and as far inland
as the Northern Rockies. Snowfall in the Cascades, Olympics, and
into the typical northern California ranges will not only have the
aid of strong 250-500mb upper level divergence Friday morning=20
ahead of the trough, but some upslope enhancement will also help in
produce heavy snow at elevations generally above 3,500ft. Farther
east, the Lewis Range an Idaho Panhandle will not only have the
anomalous moisture and synoptic-scale ascent to work with Friday
evening, but a strong 1040+mb high in the Canadian Prairies to aid
in upslope ascent via low-level easterly flow. Snow should
gradually taper off in the northern Rockies by Saturday afternoon.

One other area of interest is the Columbia Basin and Columbia Gorge
where cold air spilling in through the valleys to the north will
allow for a CAD signature to take shape on Friday. Some minor ice
accumulations may occur as far east as the Salmon River and
Sawtooth Mountains, but it is the Columbia Basin and Columbia River
Gorge that sport low chances (10-30%) for ice accumulations >0.1"
through Friday afternoon. There will be a lull between storm
systems for parts of Washington on Saturday morning, but the ORegon
Cascades will feel the pesky presence on onshore flow that leads to
more high elevation mountain snow throughout the day. By Saturday=20
night, the next Pacific storm system slams into the Olympic=20
Peninsula with more >4,000ft heavy snowfall for the Olympics and=20
Cascades which should weaken in intensity by Sunday afternoon.=20

Over the course of the next three days, the higher elevated/more
remote areas of the Cascades, Blue, Sawtooth, Salmon River,
Bitterroots, Lewis Range, and Tetons show high chances (>70%) for
snowfall >8". The ranges with moderate-to-high chances (50-70%) for
>12" are the Cascades and Tetons where they will; contend with a
combination of the best synoptic-scale ascent, upslope ascent, and
IVT placement. Expect some treacherous travel conditions in complex
terrain in areas where roads/passes are not closed for the season.


...Great Lakes and Northeast...=20
Days 1-3...

The strengthening -NAO block over Greenland and the North Atlantic
is in effect causing a traffic jam for the deep upper low over
Quebec. It has no where to go at the moment, thus keeping
cold/cyclonic flow in place over the Great Lakes for the next=20
couple of days. As this 500mb gyre spins over eastern Canada,=20
500mb shortwave disturbances will traverse the Great Lakes Friday=20
and Saturday. The Great Lakes will gradually cool as a result, but=20
the lapse rates will remain steep enough over and downwind of the=20
lakes to where lake-induced instability could top 500 J/kg on=20
occasion. The snow belts of the Michigan U.P., northern portion of=20
Michigan's Mitten, from northeast Ohio on north eats along the=20
Chautauqua Ridge, and from the shores of eastern Lake Ontario to=20
the Tug Hill will be most favored for heavy snow through early=20
Sunday AM. Snow will be measured in feet from northeast OH,=20
northwest PA, and western NY (south of Buffalo) to the Tug Hill and
even as far south as the Syracuse metro area. Snowfall rates in=20
these areas of 1-2"/hr are expected with >3"/hr rates possible in=20
the bands hitting the Tug Hill and central NY overnight. The WSSI=20
shows Major Impacts for these areas with even a small Extreme area=20
(extremely dangerous to impossible travel and widespread closures)=20
in the Tug Hill through Sunday AM.=20


...Ohio Valley, Central Appalachians, and Mid-Atlantic...=20
Days 1-2...

A compact wave of low pressure moving across Iowa this morning
will move progressively eastward beneath a modest shortwave racing
southeast within the broad trough amplifying across the eastern
CONUS. Immediately in the wake of this shortwave, a potent jet
streak reaching 150kts will dig rapidly out of the Northern Plains,
producing a narrow but intense corridor of overlapping ascent
through height falls, PVA, and LFQ diffluence. This will help at
least subtly deepen the low and the accompanying
baroclinicity/attendant fronts as it moves from near Quad Cities,=20
IA tonight, eastward to off the VA coast Friday night.

Downstream of this wave, modest isentropic ascent along the
285-290K surfaces will cause a rapid uptick in column RH, with the
accompanying WAA leading to a deepening of the DGZ and modest
925-850mb fgen. This will result in a expanding area of=20
precipitation falling as snow across the OH VLY into the Central
Appalachians tonight, with moisture spilling across the
Appalachians into the Mid-Atlantic Friday afternoon. The system is
compact and progressive, so snowfall in general across the OH VLY
should be modest reflected by WPC probabilities for 2" that are
generally just 10-30%, but locally enhanced snowfall rates of=20
1"/hr through any fgen band could cause lollipops of higher=20
snowfall exceeding 3 inches.

The heavier accumulations are likely in the Central Appalachians
where, despite the modest forecast depth of the DGZ,
appreciable upslope flow will drive ascent favorably into the DGZ,
while some moisture enhancement occurs on post-frontal NW flow from
the Great Lakes. This suggests an extended period of moderate to
heavy snow, especially Friday morning through evening, with the
higher elevations receiving the most significant snow
accumulations. Current WPC probabilities are high (<90%) for more
than 4 inches D1, falling to 30-50% for continuing snowfall D32.
Event total snow here could reach double-digits in the higher
terrain.

Finally, although total accumulations are expected to be minimal, some
snow showers or convective snow showers moving along the cold front=20
may crest the Appalachians and move across the Mid-Atlantic states
Friday afternoon as the vorticity max dives southeastward. The=20
late afternoon timing of this vort max allows for steep lapse rates
from the sfc all the way up into the DGZ, which could promote brief
snowfall rates above 1"/hr as shown by low probabilities in the WPC
snowband tool. Low-level temperatures will be slightly above=20
freezing from Richmond, VA to Philadelphia, PA, but wet-bulb
temperatures below 0C could allow any heavier rates to accumulate
on roads and elevated surfaces, causing hazardous travel despite=20
minimal accumulations, during the Friday aftn/eve commute.
Additionally, where low-level instability is the greatest,
especially overlapping the higher moisture content focused along
the MD/PA line, isolated snow squalls are also possible.


...Northern High Plains...
Days 1-3...

An amplified upper level trough moving into the Pacific Northwest
Friday evening will spread downstream ascent through increasing
divergence into the Northern High Plains by Saturday morning. This
feature is progged to split into dual vorticity lobes, with the
primary one swinging into the Central Rockies, while a secondary
impulse lifts into Alberta/Saskatchewan. Downstream of this trough,
weak shortwave ridging will initially be placed over the region,
but this will gradually become displaced by the approaching trough,
leading to increasingly strong deep-layer ascent. This forcing will
act upon a saturating column as 290K isentropic ascent maximizes=20
in a narrow corridor from NW to SE from the foothills of central MT
east into the High Plains of SD, which will effectively overlap
with a stripe of robust 700-600mb fgen. The overall setup is not=20
ideal for extremely heavy snow, but an expanding footprint of=20
snowfall is likely, and rates will be enhanced by a cold column=20
with very fluffy SLRs expected. Additionally, the guidance has
become more excited about snowfall potential as reflected by an EFI
of 0.8 to 0.9, primarily due to an extended period of moderate to
at times heavy snow. WPC probabilities have increased as well, and
now feature a 70-90% chance for 4+ inches across central MT, with
locally up to 10 inches possible in some of the higher terrain, and
a 30-50 chance for 4+ inches across far NW SD.=20


...Central Plains through the Mid-Atlantic...
Day 3...

Guidance is becoming more aligned this morning, suggesting high
confidence in a significant winter storm developing across the
Central Plains late Saturday night and then strengthening while
moving east towards the Mid-Atlantic on Monday. While there
continues to be considerable temporal spread in the models, the
overall evolution and footprint is quite similar, and significant
impacts due to snow, sleet, and freezing rain are expected.

Saturday afternoon, a deepening shortwave will eject from the
Central Rockies, moving into the Southern Plains by Sunday morning
while taking on a negative tilt. At this point there is already
considerable differences in the timing and intensity of the upper
low, with the EC and accompanying ensemble the fastest, the
GFS/GEFS making up the slow end of the envelope, and the CMC/CMCE
in the middle. That being said, the EC has been the most
consistent, and the other guidance has trended faster, but the
500mb forecast trends from all guidance have gotten deeper in the
past few cycles, suggesting maybe the EC is a bit too fast.
Regardless, significant spread in timing and intensity results in
lower than typical confidence for D3.

Despite uncertainty in timing and placement, confidence has
increased dramatically in an impactful winter storm. Regardless of
the exact placement, the setup is favorable for a widespread winter
storm from Kansas through the Ohio Valley D3, then shifting into
the Mid-Atlantic D4 /beyond this forecast period/. As the upper low
tilts negatively and then closes off, a subtropical jet streak will
sharpen and rotate around the base of this trough leading to
coupled downstream ascent. At the same time, increasing downstream
moist advection on low-level flow from the Gulf of Mexico will
rapidly saturate the column as isentropic ascent along the 295K=20
surface lifts northward and then pivots cyclonically into a TROWAL=20
around the developing cyclone.=20

As this low intensifies, in conjunction with the strengthening=20
WAA, the setup seems to conceptually support a laterally=20
translating band of heavy precipitation from west to east. With=20
DGZ depth probabilities rising to above 30% for 100mb of depth, and
a an overlap of folding theta-e with -EPV in cross sections
supporting CSI/CI, an impressive band with snowfall rates at least
1-2"/hr appears likely. Additionally, a secondary axis of=20
deformation could lengthen the heavy snow in some areas with=20
more impressive snowfall rates.

There is still uncertainty as to where the transition zone between
heavy freezing rain, sleet, and snow will occur, and guidance has
trended a bit north today, but the EFI is already highlighting=20
values nearing 0.9 with an SoT of 2 from eastern KS through=20
southern IN, and the WSSI-P is suggesting a greater than 70% chance
for moderate impacts in this same geographic window.

In addition to impacts, although confidence in exact placement is
uncertain, WPC probabilities current reflect a high risk (>70%) for
8+ inches near the NE/MO/KS borders, with a stripe of high
probabilities for 4+ inches extending as far east as Indianapolis
and as far west as central Nebraska. On the south side of the
system, WPC probabilities indicate a 50-70% chance for 0.1" in a
stripe from eastern KS through southern MO and into southern IL,
with locally 0.25+ inches becoming more likely across parts of this
area.

This system has prompted the issuance of key messages which are=20
linked below.


Weiss/Mullinax



...Winter Storm Key Messages are in effect. Please see current=20
 Key Messages below...

https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late=
stKeyMessage_1.png__;!!DZ3fjg!9KOzX81tAmUNOqiKCx8P8OiXBQFhK-sztseVXbHwWL81L=
TvK5Q5kgTuibfWT_jWBSaeL-Ch4jqpuEPW-El87JRtFxr4$=20

$$

=3D =3D =3D
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